Electrically writable and rewritable EEPROMs find extensive applications because it allows on-board rewrite and rewrite in units of pages or bytes. In recent years, therefore, manufacture methods for forming fine tunnel windows on FLOTOX-TYPE EEPROM have been proposed in Patent Documents 1 to 3 and the like.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-027406    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-183763